Presently, introduction of carrier aggregation using multiple frequency bands is being promoted. It is envisaged that multiple RF (Radio Frequency) circuits are provided in user equipment supporting the carrier aggregation. It is envisaged that since the RF circuits specific to the respective frequency bands are provided in the user equipment, the user equipment can perform inter-frequency measurement without need of configuring measurement gap.
Meanwhile, it is discussed that the user equipment supporting the carrier aggregation with multiple frequency bands is implemented in a single RF circuit. By implementing the user equipment in the single RF circuit, some effects such as reduction in power consumption or fabrication costs are expected. In this case, the measurement gap must be configured for the user equipment to perform the inter-frequency measurement, and the implementation is discussed in the course of standardization.
As illustrated in FIG. 1, the length of the existing measurement gap is typically set to 6 ms, and the user equipment is not allowed to receive downlink data in that period. In addition, if a period for HARQ (Hybrid Automatic Repeat Request) feedback of ACK/NACK to be returned after 4 ms for transmitted data is reserved, the period of 4 ms preceding the measurement gap cannot be also substantially used to transmit the downlink data. This is similar to uplink data, and for example, in cases of FDD, the user equipment is not allowed to transmit the uplink data in a period of 6 ms and a period of 1 ms after the measurement gap.
As one solution to shorten the unavailable transmission period, small gap measurement is proposed. In the small gap measurement, as illustrated in FIG. 2, in the first and last two subframes (small gap period) of the existing measurement gap, the user equipment interrupts communication with a camped base station so as to measure other cells targeted for the inter-frequency measurement and performs preparation operations for the inter-frequency measurement such as adjustment of the RF circuit. The user equipment not only performs the inter-frequency measurement but also receives downlink data from the camped base station in subframes sandwiched between the two subframes.
See 3GPP R4-140766 “Discussion on interruptions in measurement without gaps” in details, for example.